Add data to http response using rxjs

I have a trip entity which contains driver id. I can get fetch trip using RESTFull endpoint, e.g. /trips/2/.

//example response
{
  id: "2",
  driver_id: "123"
}

I can fetch driver details using the endpoint. e.g. /drivers/123/, My final respected response is

//expected response from observable
{
  id: "2",
  driver_id: "123",
  detailed_driver: {
    name: "abc",
    id: "123"
  }
}

Currently I do it as follows

this.http("/trips/2/").map(data => data.json()).subscribe(trip => {
   this.http("/drivers/" + trip.driver_id + "/").map(data => data.json()).subscribe(driver => {
      trip.detailed_driver = driver;
      this.trip = trip 
   }
}

How can I use Rxjs to use these two endpoints to get final expected response from a single observable?